On the Water

Swell information — Part I

In the last few months, the National Weather Service has made a small but noticeable change in the information sent to mariners via their regularly issued marine weather forecasts for the coastal waters of the U.S. East Coast.

The change involves swell information, both the direction and period. It has been routinely added to the normal wave forecasts, both in the online text forecasts and radio broadcasts. This is a significant positive step.

For those unfamiliar with swells, they are waves, typically somewhat smooth and rounded in appearance, that are generated by non-local winds. They can travel moderate to very long distances. Swells are distinct from local wind waves (the “normal” waves in your forecast), which are formed and driven by local winds and can change direction rapidly as the local wind direction and velocity changes. Swells, too, will change direction and size, but generally much more slowly than local wind waves do. In combi-